This invention relates to optical pattern tracing controls and in particular, circular scanning non steering controls, the type described in U.S. Pat. No. 3,883,735 issued May 13, 1975 to Francis P. Murphy et al.
Optical pattern following devices which depend on circular scanning optics of the type described in the foregoing patent tend to have errors as they pass around corners caused by the difference between the point of rotation and the point of actual scan of the pattern. This difference, referred to as lead, is essential to the operation of the system. The amount of lead required depends upon the speed of tracing. The greater amount of lead provided, the greater tendency to undershoot at corners. In the system described in the foregoing patent, a circular scanner produces a pair of sinusoids which represent the position of the scanner, in its circular scan, as a sine wave and a cosine wave, and a sensing pulse representing the instant when the scan crosses the pattern. These signals are combined in sample and hold circuits, and sample the sine wave and cosine wave at a time determined by the traverse of the pattern by the scan. As has been previously described in U.S. Pat. No. 3,883,735, a sample and hold circuit generally consists of means to produce a sample pulse, which is in this case the sensing pulse indicating the traverse of the pattern by the scan, and uses it to sample the sine wave or cosine wave. A sample and hold circuit normally samples the amplitude of the sine or cosine wave at a time determined by the sample pulse and stores the value in a capacitor. By selection of the capacitor and the width of the sample, it is possible to produce the effect of a low pass filter by requiring that several samples be applied to the capacitor before substantial change in potential is produced. This function has been disclosed and claimed in U.S. Pat. No. 3,727,120.